The present invention relates to a polishing apparatus for polishing a plate-like article such as a semiconductor wafer or an optical lens.
In manufacturing integrated circuits, optical devices and the like, it is important that a workpiece used to form an integrated circuit, optical device or the like have a highly planar surface. Recently, a polishing technique referred to as xe2x80x9cChemical Mechanical Polishing (CMP)xe2x80x9d has been commonly employed in polishing semiconductor wafers. In Chemical Mechanical Polishing, a semiconductor wafer held by a rotating carrier body is brought into contact with a polishing surface provided on a rotating turntable while an alkali or acid abrasive slurry is supplied onto the polishing surface, whereby the wafer is polished both mechanically under relative motion between the rotating carrier body and the rotating turntable, and chemically by the supplied slurry. In this way, the wafer surface can be polished to an exceptionally high degree of flatness. However, it is important that the surface of the carrier body which is used to hold and press the wafer against the turntable polishing surface also be uniformly flat, and that a force applied in pressing the wafer against the polishing surface be applied across the surface of the wafer uniformly. It has been difficult to meet this requirement.
One method which has been developed involves covering the surface of a carrier body with a resilient membrane and forming a pressurized fluid chamber between the inner membrane surface and the carrier body surface. In a polishing operation, a wafer held on the outer surface of the membrane is pressed against the polishing surface under a fluid pressure supplied into the fluid chamber to impart a uniform pressure across the wafer surface. To hold the wafer on the membrane surface during transfer to and from the polishing surface, a negative pressure is generated between the wafer and the membrane by making the latter surface concave. However, this method is subject to a problem that when a negative pressure acts on a wafer causing it to bend, if even minute defects exist on the wafer""s surface, the wafer may break. A solution to this problem has been desired.
The object of the present invention is to provide a polishing apparatus in which a workpiece such as a semiconductor wafer or the like is securely held on a carrier body surface under a suction force. This suction force is applied during transfer of the workpiece to and from a polishing surface, and when it is brought into contact with the polishing surface a positive pressure is provided which acts across the surface of the workpiece uniformly, whereby a high degree of flatness of the workpiece surface can be attained.
According to one aspect of the present invention, a polishing apparatus comprises a carrier for holding a plate-like workpiece and bringing it into contact with a polishing surface. The carrier comprises a carrier body having a flat surface and one or more recesses formed in the carrier body surface, a non-porous inner sheet provided adjacent to the carrier body surface and covering it, an outer sheet covering the inner sheet, with openings extending from the inner to outer surface of the outer sheet, and a pressure supply device for generating selectively a negative or positive pressure between the inner sheet and the carrier body surface. The inner and outer sheets are sealingly connected to the carrier body along their outer peripheral edges. When a negative pressure is generated between the inner sheet and the carrier body surface, a suction force acts on the outer surface of the outer sheet to draw a workpiece against it. If a positive pressure is provided, it acts on the workpiece through the inner and outer sheets and presses it against the polishing surface.
Since the inner sheet is made of a flexible material and the outer sheet of a relatively rigid material, when a negative pressure is provided, it causes a portion of the inner sheet to deform, and this deformed portion enters the recesses in the carrier body surface, with the result that spaces are formed between the deformed portions of the inner sheet and the outer sheet. In these spaces, a secondary negative pressure is generated and transferred through the openings formed in the outer sheet to the outer surface of the outer sheet to draw a workpiece against this outer surface. In contrast, if a positive pressure is provided, it acts to urge the inner sheet against the outer sheet thereby pressing a workpiece held on the outer surface of the outer sheet against the polishing surface.
The polishing apparatus further includes a negative pressure or vacuum supply device adapted to be fluidly communicated with the spaces formed between the deformed portions of the inner sheet and the outer sheet. Specifically, the carrier body is provided with a circumferential surface extending from the carrier body surface at a predetermined angle in a direction away from the polishing surface when the carrier is positioned for polishing a workpiece, and the circumferential surface is provided with a passage having one end adapted to be connected to the negative pressure supply device and the other end fluidly connected to the recesses formed in the carrier body surface. The negative pressure provided by the negative pressure supply device enables the workpiece to be held securely against the outer sheet.
Further, a pressure ring is provided around the carrier in such a manner as to be able to move vertically independently of the carrier to press against the polishing surface. Preferably, the pressure ring depresses the polishing surface to the same level as that of an area of a workpiece to be polished to avoid an excessive force being imposed on the edge of workpiece when the rotating polishing surface comes into contact with the rotating workpiece.
These and other features and advantages of the present invention will become apparent from the following detailed description of the invention when taken in conjunction with the accompanying drawings wherein like reference numerals designate like or corresponding elements throughout the several views.